1. Field of the Invention:
The present invention relates to an optical recording and reproducing apparatus for optically recording a signal on a recording medium and/or reproducing the signal which has been recorded on the recording medium by using a light source such as a laser. More specifically, the present invention relates to a tilt control apparatus having a tilt servo system for maintaining an orthogonal relationship between the optical axis of a light beam used for recording and reproducing information and the information recording surface of a recording medium (or disk).
2. Description of the Related Art:
A conventional tilt control apparatus is known from Japanese Laid-Open Patent Publication No. 61-51630 (optical head apparatus), for example. As disclosed in this patent publication, a conventional tilt control apparatus detects an error in the orthogonal relationship between a disk and the optical axis of a light beam irradiated onto the disk (the "orthogonal relationship" herein refers to the state where the disk falls at right angles with the optical axis of the light beam irradiated on the disk) by using a sensor exclusively used for detecting the error, i.e., a tilt sensor, thereby maintaining the orthogonal relationship based on an orthogonality error signal detected by the sensor.
Hereinafter, a conventional tilt control apparatus will be described.
FIG. 41 is a block diagram showing a schematic configuration for a conventional tilt control apparatus. In FIG. 41, an optical disk 101 is placed on a rotation axis of a spindle motor 111. An optical pickup 102 includes: a light-emitting element such as a laser; an objective lens; and an objective lens actuator and irradiates a condensed light beam onto the optical disk 101. A light-receiving element is further provided for the optical pickup 102, thereby detecting an information reproduced signal RF and a servo signal including a focusing error signal FE and a tracking error signal. A signal corresponding to the recorded information, i.e., an information reproduced signal RF is obtained by converting the light reflected by the optical disk 101 into an electric signal. A focusing error signal FE is an electric signal corresponding to the distance between the focal point of the condensed light beam and the information recording surface of the optical disk 101. This focusing error signal FE is fed back to the objective lens actuator of the optical pickup 102 via a focusing controller (not shown), so that the focusing error signal FE is controlled to be approximately zero.
In the state where the above-described focusing control has been performed, a tracking control (not shown) is performed so that the optical pickup 102 traces the tracks on the optical disk 101. Then, the information recorded on the optical disk 101 can be read out based on the reproduced signal RF.
However, if an error is caused in the orthogonal relationship between the optical disk 101 and the optical axis of the light beam 103 irradiated onto the optical disk 101, then the reproduced signal RF is erroneously identified more frequently. Therefore, a tilt servo operation is required to be performed for controlling so that the orthogonal relationship between the optical disk 101 and the optical axis of the light beam 103 irradiated onto the optical disk 101 is maintained.
Next, the tilt servo operation will be described. A tilt sensor 107 for detecting the orthogonal relationship between the optical disk 101 and the optical axis of the light beam 103 irradiated onto the optical disk 101 is mounted on the optical pickup 102. The tilt sensor 107 includes a light source such as a light-emitting diode for emitting light toward the optical disk 101 and a light-receiving element for receiving the light reflected by the optical disk 101. A signal output from the tilt sensor 107 is input to an orthogonality error detector 108, so that an orthogonality error signal is generated therefrom. The optical pickup 102 is mounted on a tilting mechanism 110. A tilt controller 109 makes the tilting mechanism 110 vary the tilt angle of the light beam to be irradiated onto the optical disk 101 with respect to the optical disk 101 based on the orthogonality error signal, thereby controlling so that the error in the orthogonal relationship becomes zero.
However, the conventional tilt control apparatus described above has the following problems. Specifically, in the case of performing a tilt servo operation using the tilt sensor 107, an error is adversely caused in the orthogonal relationship between the optical disk 101 and the optical axis of the light beam 103 irradiated onto the optical disk 101 in spite of the tilt servo operation if the characteristics of the tilt sensor 107 are varied because of the variation in the solid state properties thereof or if an error has been caused in assembling the devices.
That is to say, since an offset has been generated, a normal orthogonal relationship cannot be maintained even if the orthogonality error signal is zero. The offset of the orthogonality error signal must be adjusted to be a predetermined value or less for the respective devices during the assembly process, so that the production cost is disadvantageously increased.
In addition, if an output from the tilt sensor 107 is varied because of the degradation in the characteristics of the tilt sensor 107 with the passage of time or the variation in the temperature, an offset is caused in the orthogonality error signal. As a result, an error is adversely caused in the orthogonal relationship between the optical disk 101 and the optical axis of the light beam 103 irradiated onto the optical disk 101 even if the tilt servo operation has been performed. As described above, if an error is caused in the orthogonal relationship, then the reproduced signal RF is erroneously identified more frequently, so that the recording and reproduction operations cannot be performed under optimal conditions any longer.